1. Field of the Invention
This invention relates to the field of medical electronics and particularly to cardiac pacer leads adapted to be formed in a variety of configurations by a relatively simple method, at the time of implantation.
2. State of the Art
Electrical stimulation of body tissue and organs is a method of treating various pathological conditions which is becoming quite commonplace. Such stimulation generally calls for making some type of electrical contact with the body tissue or organ. In particular, with respect to the heart, electrical pacer leads are physically coupled or implanted into the myocardial or endocardial tissues.
In order to attach the output electrodes of a heart pulse generator or pacer to the heart, it is necessary to make an incision in the neck whereby the leads of an external or an implanted pacer may pass through the patient's subclavian vein and into the patient's heart. The leads used for such purpose typically have long, thin, flexible conductors, typically of a helical configuration, enclosed by an insulating material of a character suitable for implantation within the patient's body. At the distal end of such leads, the flexible conductor is connected to an electrode capable of being placed in contact with the patient's heart. In some lead configurations, typically known as "bipolar leads", two such flexible conductors are included within one insulating covering, thereby providing a forward conduction path and a return path for electrical stimulation to the heart. For this type of lead, distal and proximal electrodes are provided, spaced from each other in order to make contact with separate portions of the heart, e.g. the atrium and ventricle of the patient's heart. To this end, the lead is inserted through the patient's subclavian vein and into his heart, with the distal electrode coming to rest in the ventricle cavity. The distal electrode so inserted normally will be confined to the ventricle without difficulty. However, as noted by U.S. Pat. No. 3,729,008, the proximal electrode for stimulating the atrium has difficulty in maintaining its position within the confines of the atrial cavity. In this regard, the atrium has smooth walls, allowing easy slippage of an ordinary linear electrode. U.S. Pat. No. 3,729,008 suggests placing a curve in the pacer lead of a shape and dimension to conform the lead to the atrium and to be suitable for gripping the moving atrium wall, whereby good electrical contact is maintained between the proximal electrode and the undulating, flexible atrium wall of the patient's heart. Further, the noted patent suggests that the pacer lead be made of a medical-grade, resilient rubber that is formed by a process such as vulcanization or injection molding. It is recognized that such medical-grade, silicon rubber is selected in order to provide a material resistant to corrosion by the body fluids to the patient in which such lead is implanted.
In U.S. Pat. No. 3,596,662 of Bolduc, assigned to the assignee of this invention, there is disclosed a method of molding a pacer lead having electrical conductors encapsulated in a unitary, homogeneous body of flexible, non-conductive plastic material, such as silicone rubber. This process of forming the pacer lead utilizes a 3-piece mold comprising a pair of identical female molds and a male mold, the female molds having elongated cavities of a size and shape of one-half of the pastic body covering the conductors. The male mold has elongated ribs, each of which has a volume equal to one-half of the volume occupied by the conductors, plus one-half of any volume to be left void. Halves of the pacer leads are initially made by mounting the male mold in an assembled relation with the female mold and injecting the silicone rubber into the mold cavity under high pressure. The male mold then is removed from the female mold, exposing the grooves in the molded halves of the silicone rubber bodies. Thereafter, the conductors are placed between the halves of the bodies in alignment with the grooves formed by the ribs of the male mold. The conductors are encapsulated in the body by combining the halves of the body into a one-piece member by simultaneously subjecting the assembled female molds to a high pressure and temperature sufficient to cause the mating halves of silicone rubber to adhere to each other.
The use of such a method of forming pacer leads has proved very advantageous, especially with the use of linear or straight pacer leads. However, it is desirable to form such pacer leads with a variety of configurations in order to permit the retention of the lead's electrode within the patient's heart and in particular, to permit the resilient placement of the atrial or proximal electrode against the inner flexing wall of the patient's atrium. It has been contemplated to adapt the method as taught in U.S. Pat. No. 3,596,662 to the formation of such curved pacer leads. However, the formation of the 3-piece molds in such a non-linear configuration would be unduly expensive in that the machining of such molds into the desired curves would be expensive and time-consuming. Further, it is contemplated that in the course of the surgical procedure by which the heart pacer and pacer lead are inserted within the patient, the attending surgeon may find it desirable to form the pacer lead into one configuration or another or to vary the size of a particular configuration in order to adapt the pacer lead to a particular patient. If a pacer lead made of a conventional silicone rubber coating is used, such forming of the pacer lead at the time of implantation would be impractical in that once cured, the silicone rubber is set in its normal, linear configuration and would not maintain the desired configuration.